interface A {
  double f(double x, double y);
}
class B implements A{
  public double f(double x, double y){
    return x*y;
  }
  int g(int a, int b){
    return a+b;
  }
}

public class xiti6_1{
  public static void main(String args[]) {
    A a = new B();
    System.out.println(a.f(3,5));
    B b = (B) a;
    System.out.println(b.g(3,5));
  }
}
